1. Field of the Invention
The present invention relates to a hydraulic pumping system particularly for pumping fluids at low pressure and in particular for pumping petroleum fluids at the bottom of a well.
2. Description of the Prior Art
Various methods and devices are used in the field of hydrocarbon production for pumping low-pressure fluids.
Classical hydraulic pumping by jet or piston type bottom-hole pumps requires, for example:
either lifting the drive fluid mixed with the product through the annular gap between the casing and the tubing, or through the central tubing, depending on the method of hydraulic circulation chosen. The drive fluid is, for example, the water in the deposit or a degassed, processed crude that may contain additives and/or solvents, avoiding problems of fouling, emulsion, or rusting. One of the drawbacks of this method is that the mixture of drive liquid and product fluid can lead to cross-pollution of these two fluids. This option demands a voluminous and expensive processing facility at the surface to filter and recycle the drive fluid; PA1 or using a well completion with an extra tubing for lifting the expanded drive fluid, which is an expensive and complex option when reduced to practice. PA1 .rho..sub.M is the density of the drive fluid, PA1 g represents the local gravity constant (approximately 9.8 m/s.sup.2) PA1 h is the pump depth PA1 P.sub.suc is the suction pressure of the fluid in the deposit, PA1 S is the section of the bottom-hole hydraulic pump piston. PA1 the first part or drive part has the pressure-reducer and can be provided with means for introducing drive fluid, with the drive fluid also playing the role of buffer fluid, PA1 the second part has for example a means playing the role of a piston and defining two variable-size chambers, one of the chambers being in communication with the pumped fluid introduction and a discharge and the other communicating with the drive fluid introduction pipe. PA1 the system is simple to build and operate, PA1 the check valves can advantageously be disposed at the upper part of the pump to favor initial expulsion of the free gas in the discharge phase of the pump, which improves pumping efficiency, PA1 the dead volumes at the intake can be minimized, PA1 since the risk of injecting substantial amounts of drive liquid into the deposit in the pumping phase is reduced, it is possible to dispense with a valve of the standing valve type normally used in classical hydraulic pumping, PA1 the assembly comprised of the pressure reducer and the bottom-hole pump can be installed in various ways, for example by suspending it from a coil tubing paid out from the surface, or by lowering it from an enclosure at the surface to the bottom simply by gravity inside a tubing of sufficient diameter, according to the technique of free hydraulic pumps. PA1 since the seals of the double jack and the pump are in contact with the drive liquid itself, the lifetime of the system is improved, PA1 the mechanical forces applied to the pump are minimized because the two pump chambers containing the drive liquid and the product are almost at the same pressures in the discharge phase and in the suction phase, PA1 it is not necessary to align the pump exactly coaxially in the double jack, which facilitates setting up the system, for example in the event of poor-quality completion.
The present invention injects and recovers the drive fluid through one and the same pipe, alternating the drive fluid injection and removal phases in regular cycles. Using two different bottom/surface hydraulic links for the drive fluid and the pumped fluid prevents mixing of the drive fluid and product fluid during the pumping operation.
To implement the suction phase of the bottom-hole pump, the pressure generated at the bottom by the drive fluid is reduced so that it is below the well pressure at right angles to the pump suction.
Various methods have been described in the prior art for producing this pressure drop.
A first solution, described for example in U.S. Pat. Nos. 2,519,679, 3,941,510 and 4,405,891, consists of using a light drive fluid such as a liquid or a gas.
However, the use of low-density liquids (liquefied butane or propane, alcohol, etc.) does not produce a sufficient pressure for classical applications. The use of gas (natural gas or nitrogen) has the drawback of requiring substantial compression work with each cycle, leading to a very low energy efficiency and a very slow cycling rate.
A second solution, referred to for example in U.S. Pat. Nos. 2,180,366, 3,420,183, and 4,616,974, consists of assisting the suction phase of the pump by having the column of drive fluid and column of pumped fluid (assumed to be a liquid monophase) work alternately. This solution requires complicated machinery at the bottom and at the surface, comprising an assembly of check valves, pistons, and cylinders with different cross sections. This technique, which enables any drive fluid such as water to be used, is in this case well-suited for pumping water. On the other hand, production of crude with free gas, which represents a general application case of oil production, would require considerably increasing the volume of drive fluid transferred with each cycle to assist lifting the product by compressing the product gas, thus considerably limiting energy efficiency and production rate. Fitting a gas separator, whose efficiency is imperfect, to the suction end of the bottom-hole pump would complicate completion of the well without entirely eliminating this drawback.
Finally, a third solution, as described in U.S. Pat. Nos. 2,555,613 or 4,013,385, consists of using a mechanical or pneumatic spring directly applying an upward return force to the piston of a classical piston-type bottom-hole hydraulic pump. This solution faces the great difficulty of installing a long, powerful spring, which is necessary for substantially reducing the hydrostatic load produced by the drive fluid column, in a small-diameter space. The force P to be applied to resist this column must meet the condition: EQU F&gt;(.rho..sub.M gh-P.sub.suc)S
where:
The force P thus calculated would frequently exceed 1000 kg.